1. Field of the Invention
This invention relates generally to the sampling of petroleum and petroleum products, and, more particularly, to an apparatus for simultaneously sampling a stream of petroleum or petroleum products in at least two positions within the stream.
2. Description of the Related Art
During the refinement of petroleum and petroleum products, process engineers use various test methods to determine the physical and chemical characteristics of the products. These physical and chemical characteristics indicate whether the products are being processed as the engineers had planned. Quite often, the test results point to problems in the refinement process, such as corrosion or the improper mixture of chemical additives. In addition, the test results are also used for custody transfer and pricing determinations.
For the process engineers to make accurate determinations in view of the test results, the samples should be representative of the material or product being sampled. Whether the samples represent the sampled material depends upon the material being sampled; the tank, carrier, container or line from which the sample is obtained; the cleanliness of the sample container; and the sampling procedure used. Ideally, process engineers obtain a sample from various locations in the tank or container so that the samples will be representative of the material being sampled. Designation D 4057-81 of the ASTM standards describes various sampling methods and apparatuses which may be used in an effort to achieve these representative test results.
To sample a semiliquid material in a pipeline, a filling line, or a transfer line, process engineers prefer to use a continuous automatic sampling method. One preferred automatic sampling method employs a sampling probe that is inserted into the stream flowing within the pipeline. Ideally, the probe is inserted into a portion of the stream that is representative of the entire stream. FIGS. 1-3 illustrate various commonly used probe designs. FIG. 1 illustrates a tube 10 that extends to the center 14 of the pipeline 12. The tube 10 terminates at a bevelled angle, typically a 45.degree. angle, so that the open tube 10 faces upstream. FIG. 2 illustrates a tube 18 having a bent end 20. The tube 18 is positioned with the end 20 at the center of the pipeline 12, so that the opening of the tube 18 faces upstream. FIG. 3 illustrates a closed-end tube 22 having a round orifice 24 spaced near the closed end. The tube 22 is positioned such that the orifice 24 is in the center of the pipeline 12 and facing upstream.
While the probes illustrated in FIGS. 1-3 were shown positioned in the center of the pipeline, a sampling probe cannot always be positioned in the center and still obtain a representative sample of the material in the stream. If a nonhomogeneous fluid is to be sampled, the location, position, and size of the probe may differ to minimize any separation of water and heavier particles that would make their concentration in the sample different from their concentration in the stream as a whole. In addition, section 9.4.3 of the above-mentioned ASTM standards lists a number of other considerations regarding probe location. For instance, the standards recommend that the probe should always be in a horizontal position to prevent drain-back of any part of the sample into the main stream. Preferably, the sampling probe should be located in a vertical run of pipe. However, if a vertical run is not available, a probe may also be located in a horizontal run of pipe provided the flowing velocity is high enough to provide adequate turbulent mixing. While adequate flowing velocity may not eliminate a concentration difference between the bottom of the pipe and the top of the pipe, it may provide an average concentration at the center of the pipe that will be representative of the concentration in the entire stream.
Of course there are other similar methods of retrieving a fluid sample from a pipeline. For instance, an outlet pipe can be connected in fluidic communication with the main pipeline. A valve controls the fluid flow through the outlet pipe. When a process engineer desires to sample the material within the main pipeline, the engineer places a container at the end of the outlet pipe, and opens the valve so that a sample flows from the main pipeline, through the outlet pipe, and into the container. However, since the outlet pipe does not extend into the main pipeline, the sample may not be representative of the material within the main pipeline.
In a semiliquid environment (vapor/liquid mix), it is often desirable to provide a water wash which is distributed uniformly in the pipeline to prevent the overhead of vapors from causing corrosion problems within the pipeline. In this situation, it is desirable to take a sample at various positions across the width of the pipeline. Therefore, a probe must be inserted into the pipeline at various locations. This is a time consuming task which requires careful measurement to be certain of the positioning of the tip of the probe.
The present invention is directed to over coming or at least minimizing, one or more of the problems set forth above.